1. Field of the Invention
The present invention relates to analysis of contamination of wafer surfaces, in particular, to analysis of a trace amount of transition metal such as iron and copper, for example, after formation of a dielectric film having a high dielectric constant in a transistor forming process in production of semiconductor integrated circuits.
2. Description of the Related Art
In semiconductor integrated circuits, there has been an increasingly greater demand for promoting high speed, low power consumption, high integration at the same time year by year. The size of an individual transistor in an integrated circuit is being reduced by higher integration year by year. In order to let an MOS (metal oxide semiconductor) field effect transistor function to a full extent, it is necessary to ensure a certain capacitance for gate dielectric film between a gate electrode and a semiconductor channel. However, in the case where the area of the gate becomes small, the certain capacitance cannot be ensured without thinning the dielectric film.
For this reason, the thickness of silicon oxide film used as gate dielectric film is being reduced year by year. However, when the dielectric film is made thin to a certain extent or more, it is impossible to ensure a desired insulation performance. Then, a material from which dielectric film is formed is replaced by a material having a higher dielectric, so as to achieve a configuration in which even with film having large thickness, a certain level of capacitance is ensured. At the present, the silicon oxynitride film in which a certain amount of nitrogen is contained in a silicon oxide film is predominantly used.
In order to form an even smaller transistor, materials having even higher dielectric constants have been examined. As a result of comparing and examining the electrical, chemical, and physical characteristics of various materials, hafnium oxide or a complex oxide of hafnium and silicon is about to be used.
It is well known that when a gate dielectric film is contaminated with a trace amount of metal, especially iron or copper, the metal is diffused in the film and inhibits the insulating property of the material, causing failure due to current leakage. Total reflection X-ray fluorescence is widely used by semiconductor manufacturers in order to analyze this type of contaminants with high sensitivity without destruction, and the standard analysis method has been formulated as International Standard ISO 14706. The invention solves the problems of the conventional total reflection X-ray fluorescence spectroscopy which are caused in analysis of transition metal such as iron and copper on a dielectric film (hereinafter referred to as hafnium-containing film in some cases) formed of a material containing hafnium as the main component or containing a hafnium-containing compound as the main component (hereinafter, referred to as hafnium-containing film) and serves for practical use for semiconductor manufacturers.
As described in the ISO, homogeneous X-rays having energy of W (tungsten)-Lβ 1 ray (9671 eV) or its vicinity are primarily used for analysis of transition metals on a silicon oxide film or the like. However, with conventional apparatuses of this type, it is impossible to analyze transition metal on a hafnium-containing film with a desired sensitivity for the following two main reasons 1) and 2).
1) Strong fluorescent X-rays generated from hafnium saturate a detector.
The energy of irradiated X-ray used for analysis of transition metal such as W-Lβ 1 ray is higher than that of the hafnium LIII absorption edge (9554 eV) and excites Hf (hafnium)-Lα ray with a very high efficiency. The number of hafnium atoms contained in the film is as many as about 1.4×1016 atoms/cm2, for example, in the case of a hafnium oxide having a thickness of 5 nm. On the other hand, the total reflection X-ray fluorescence spectroscopy is designed so as to detect contaminations to such a very small extent of about 1×109 atoms/cm2. When contaminants of more than 1×1013 atoms/cm2 or more are present, the fluorescent X-rays having an amount exceeding the designed limit saturate the detector, making measurement impossible. In such a case, reducing the amount of X-ray irradiated to about 1/1000 of the standard allows measurement. However, this is accompanied by a reduction in the fluorescent X-ray yields of all elements contained in the film accordingly to 1/1000, which makes it impossible to perform analysis with a desired sensitivity in a predetermined time.
2) Strong fluorescent X-rays generated from hafnium overlap Cu (copper)-K rays, which makes analysis impossible.
The very intensively generated Hf-Lα ray cause another difficulty. The Hf-Lα ray (7898 eV) have an energy that is very close to Cu—K ray (Kα: 8047 eV, Kβ: 8903 eV). The fluorescent X-ray spectroscopy is performed with a semiconductor detector having a full width of Half Maximum of about 200 eV in this energy region, and therefore in the case where the fluorescent X-ray from hafnium atoms in the number that is 107 times the number of copper atoms, which is the target for measurement for the contamination, detection is impossible.
A conventional technique using Rb-Lα ray and Si—Kα ray in order to detect Na an Al, which are impurities on a silicon wafer sample, is disclosed in Japanese Patent NO. 2843529. However, Japanese Patent NO. 2843529 discloses no technique for analyzing transition metal in hafnium-containing films.
The inventors of the invention previously proposed the following method that allows analysis of transition metal such as Cu contained as impurity in a hafnium sample in order to solve the aforementioned problems. That is, a method of selecting as the energy of irradiated fluorescent X-rays an energy that is larger than Cu—K absorption edge (8978eV), which is the target of analysis, and is smaller than Hf (hafnium)-LIII absorption edge (9554 eV) can be conceived. A configuration in which Pt (platinum)-Lα ray (9441 eV) or Ir (Iridium)-Lα ray (9173 eV), which have such an energy, is used as the characteristic ray of material that is suitably used as an anode of an X-ray tube can be conceived. The scattering ray of irradiated X-rays can be an interference factor by overlapping a trace amount of fluorescent X-rays, and therefore it is better that the energy is not too close to the Cu—K characteristic ray. For this reason, the inventors of the invention produced an X-ray tube with a platinum anode, selected its Pt-Lα characteristic ray with a monochromator, irradiated on a sample, and measured the fluorescent X-rays.
FIG. 4 is a graph showing the results of measuring the intensity of fluorescent X-rays that is obtained when a hafnium-containing film is irradiated with Pt-Lα ray.
TABLE 1FIG. 4Content31Hf-Lα fluorescent X-ray32Raman scattering of Pt-Lα ray caused by Hf33measured spectrum34synthesized peak indicating Cu-Kα expected fromcontamination of 1 × 1011 atoms/cm235Background
These measurement results confirmed that the amount of Hf-Lα ray generated was significantly small and that it became possible to measure a trace amount of Fe—Kα ray. On the other hand, it was difficult to measure Cu—Kα ray, which overlap a very large interfering X-ray. This interfering rays are Hf-Lα florescent X-ray that was still present in a certain amount and the inelastic scattering that is called Raman scattering due to hafnium atoms in the film and lost an energy corresponding to hafnium M absorption edge (1730 eV) or more by Pt-Lα ray.
Although in theory, the Hf-Lα ray can not be excited by irradiation of Pt-Lα ray, in practice, the monochromator that is realized with an analyzing crystal used has a certain energy selection properties and slightly reflects the X-rays having an energy in the vicinity of the selected energy. Since the energy difference between Pt-Lα ray and Hf-LIII absorption edge is very small such as 113 eV, when the selection energy of the mochromator is set to Pt-Lα ray components having an energy exceeding Hf-LIII adsorption edge among white X-ray components that are generated from the X-ray tube pass through the monochromator and reach the sample in very small quantity. Even if the amount of X-rays with such an energy is very small, since a large amount of hafnium atoms is present in the sample as described above, Hf-Lα fluorescent X-rays with an unnegligible intensity are still generated and inhibit detection of Cu—K ray. The amount of Hf-Lα X-rays generated can be reduced by improving the energy selection property of the monochromator. However, when it is attempted to improve the energy selection property of the monochromator, the efficiency at which X-rays with a desired energy pass tends to be also reduced, which may reduce the sensitivity of the apparatus. Thus, it was found that even with Pt-Lα ray, it was very difficult to detect a trace amount of Cu on a hafnium film.